The present invention generally relates to driving of laser diode and formation of images, and more particularly to a laser diode driver circuit for use in laser printers, optical disk drives, digital copiers, and optical telecommunication systems for driving a laser diode, as well as an image formation apparatus that uses such a laser diode driver circuit.
Conventionally, driving of a laser diode has been achieved either by a non-bias driving method or a biased driving method, wherein a non-bias driving method sets the bias current of the laser diode to zero and achieves the driving of the laser diode by a pulse current corresponding to an input signal. On the other hand, a biased driving method supplies a bias current to the laser diode with a level of the laser threshold current or less and drives the laser diode by superimposing a pulse current corresponding to an input signal to the bias current.
In the case of non-bias driving of laser diode, it takes some time after supplying of the driving current to the laser diode in response to the input signal, until there is formed carriers with a concentration level sufficient in the active layer of the laser diode for causing laser oscillation, and thus, there inevitably occurs a delay in the timing of optical emission. Thus, the use of such a non-bias driving of laser diode has been limited to the applications in which such a delay is negligible.
In the case of high-speed laser printers, optical disk drives, digital copiers, and the like, on the other hand, there is a stringent demand for high-speed driving of laser diode. When the non-bias driving is used in such applications, an optical pulse width smaller than the desired pulse width is obtained.
The biased driving method is proposed for overcoming the foregoing difficulty of non-bias driving method. In the biased driving method, it should be noted that a bias current is supplied with a level or magnitude corresponding to the threshold of laser oscillation, and the delay of optical emission is eliminated.
In the biased driving method, on the other hand, it should be noted that the laser diode emits optical radiation continuously at the level of the laser oscillation threshold (generally 200-3009 μW), even in the case the laser diode is not driven for laser oscillation. Thus, in the case the laser diode is used for optical telecommunication, there occurs a deterioration of optical extinction ratio. In the case the laser diode is used in laser printers, optical disk drives, digital copiers, and the like, on the other hand, there is caused the problem of the white background becoming dark because of the exposure to continuous optical emission of the laser diode.
In order to overcome the foregoing problems, there are proposals in the field of optical telecommunications to drive a laser diode basically with the non-bias driving method and to supply a threshold current immediately before the timing of the optical emission.
In the application of laser printers, optical disk drives or digital copiers, on the other hand, red laser diodes operating at the wavelength of 650 nm, or ultraviolet laser diodes operating at the wavelength of 400 nm, are currently used to improve the resolution of recording, while the laser diodes noted above generally require a longer time than the conventional laser diodes operating at the wavelength of 1.3 or 1.5 μm or the laser diode operating at the wavelength of 780 nm, in order that the carrier concentration level in the active layer reaches the level enabling laser oscillation. Thus, the foregoing conventional approach can only provide an optical signal with an optical width smaller than the desired optical width for the optical signal.
Further, in the case of recording low concentration images on a recording medium by an optical output continuing for a short duration such as several nanoseconds or less, there arises a problem in that the optical output power does not reach the predetermined level'needed for the beam spot. In such a case, the density of the recorded image becomes excessively thin, and the desired thickness or concentration of the image is not attained.
In order to deal with this problem, there has been a proposal in the Japanese Laid-Open Patent Publication 5-328071 to superimpose a differential pulse to the drive signal of the laser diode at the time of onset of the optical power.
However, such an approach cannot control the peak height of the differential pulses and there is a substantial risk that the laser diode may be damaged because of the uncontrolled peak height of the differential pulses. Further, this conventional approach has another drawback, in view of the fact that the duration of superimposing of the differential pulse is determined by the waveform of the differential pulse itself, in that, while it may be effective to compensate for the recording density for the initial period in which the recording density is very small, there is no guarantee that the recording density of graded images increases linearly thereafter.
Meanwhile, it is known that the relationship between the drive current and the optical output of a laser diode changes significantly with the environmental temperature. Thus, an APC (automatic power control) circuit has been used conventionally in order to maintain the optical output of the laser diode at a predetermined level. Reference should be made to the Japanese Laid-Open Patent Publication 11-298079.
An APC circuit typically includes a photodetector cooperating with the laser diode and a negative feedback control circuit, wherein the photodetector detects a part of the optical output of the laser diode and produces an electrical output signal indicative thereof, while the negative feedback circuit controls the forward bias current of the laser diode so that the electrical output of the photodiode representing the output optical power of the laser diode takes a value corresponding to the prescribed optical output level.
Thus, during a power hold interval, in which the output optical power of the laser diode is held constant, the bias current of the laser diode is controlled by the negative feedback control circuit noted above, while outside the power hold interval, a modulation signal is superimposed to the bias current so that the laser diode is turned on and off in response to the modulation signal.
According to such a construction, a fast laser modulation becomes possible, even in the interval in which the laser diode does not produce an optical beam, while such a construction has a drawback in that the laser output easily undergoes fluctuation during the interval in which the feedback control is not applied. Such a fluctuation may be caused by external disturbance such as the droop characteristics. In relation to this problem, there is proposed an APC construction in the Japanese Laid-Open Patent Publication 2-205086 for improving the accuracy and response speed of the feedback control.
Further, there is proposed an APC circuit that compensates for the decay of the output waveform of the photodetector at the time of impulse optical emission of the laser diode according to the Japanese Laid-Open Patent Publication 5-121805.
FIG. 1 shows the construction of such a conventional laser control circuit that uses an opto-electronic negative feedback loop.
Referring to FIG. 1, the laser control circuit includes a laser diode LD and a photodiode PD monitoring a part of the optical output of the laser diode LD, wherein the laser control circuit further includes a first opto-electronic negative feedback loop 2 in which there is provided a first error amplifying unit 1 controlling the forward bias current of the laser diode, such that a monitoring voltage signal Vm corresponding to a current signal Im obtained from the photodetector PD in proportion with the optical output of the laser diode in the optical-emission state (oscillating state) of the laser diode becomes identical with an external optical-emission level control signal Vc setting up the optical-emission level of the laser diode LD.
Further, there is provided a driver transistor Q1 such that the laser diode LD is connected to a collector thereof and such that a forward bias current signal of the laser diode LD is supplied to a base thereof. Further, a resistance RLD is connected across the emitter and ground of the transistor Q1.
Further, there is provided a second opto-electronic negative feedback loop 4 including a second error amplifier unit 3, such that the second opto-electronic negative feedback loop 4 controls the forward bias current of the laser diode LD in such a manner that the emitter voltage level of the driver transistor Q1 becomes equal to the extinction level control voltage (bias level control signal) at the extinction state (non-oscillating state) of the laser diode LD.
Further, there is provided a current drive unit 5 such that the current drive unit 5 switches the forward bias current of the laser diode LD between the optical-emission state and the extinction state in response to a modulation signal that provides the timing of modulation driving of the laser diode LD, and the current drive unit 5 carries out an automatic power control operation according to the value held in any of a sample hold circuit 6 and a sample hold circuit 7 respectively holding the output of the error amplifier 1 indicative of the peak value of the optical output of the laser diode LD in the optical emission state and the bottom value of the optical output of the laser diode LD in the extinction state, wherein supplying of the output of the error amplifier 1 or error amplifier 3 to the sample hold circuit 6 or 7 is controlled in response to the modulation signal via a NOR gate 9 or an AND gate 10. More specifically, the modulation signal is supplied to the NOR gate 9 or the AND gate 10 via a delay circuit 8, and the sample hold circuit 6 or 7 samples and holds the output of the error amplifier 1 or the output of the error amplifier 3 in the event the same signal state has continued in the modulation signal over a duration τ specified by the delay circuit 8.
Further, the circuit of FIG. 1 includes capacitors C1 and C2 for holding the forward bias voltage respectively for the optical emission state and for the extinction state.
Thus, according to the construction of FIG. 1, the forward bias current of the laser diode LD is controlled, in the case the laser diode LD is used to form an image in an image forming apparatus, automatically in the event the optical emission state or extinction state has continued for some time, irrespective of whether it is an image forming area or it is a non-image forming area.
Here, it should be noted that the output current Im of the photodetector PD used for monitoring the optical state of the laser diode LD is converted to the voltage signal Vm by a resistance RPD connected in series to the photodiode PD, and the resultant voltage signal Vm is fed back to the laser drive control circuit 2. In order to control the optical output of the laser diode LD with high precision, it is preferable that the monitoring electric current Im takes an output value suitable for carrying out a comparative control with respect to the optical emission level control signal Vc at the time of the feedback control.
Meanwhile, there is an increasing need of short-wavelength laser diodes in the image formation apparatuses such as laser beam printers or digital copiers so as to reduce the beam spot size and to increase the recording density of the images.
In the case of short-wavelength laser diodes, there is a tendency that the monitoring current Im of the laser diode LD becomes smaller as compared with a laser diode operating in a longer wavelength band. For example, the monitoring current Im of the a red laser diode operating at the wavelength band of 650 nm becomes smaller as compared with an infrared laser diode operating at the wavelength band of 780 nm.
In relation to this, the resistance RPD connected in series to the photodiode PD for converting the monitoring current Im to the monitoring voltage signal Vm takes a smaller resistance value in the case of the laser diode of the 650 nm band as compared with the laser diode of the 780 nm band. Thereby, there occurs a decrease of the magnitude of the monitoring voltage signal Vm in the short-wavelength laser diode similarly to the case of the monitoring current Im. Thus, such a decrease of output of the photodiode PD at the short wavelengths causes the problem of accuracy at the time of feedback control of the output optical power of the laser diode.
In view of the foregoing problems, there is a proposal to use an amplifier for amplifying the monitoring signal as represented in FIG. 2.
Referring to FIG. 2, it can be seen that there is provided an amplifier 110 for amplifying the monitoring voltage signal Vm converted from the monitoring current Im between the photodiode PD and the error amplifier 1. In FIG. 2, it should be noted that those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted.
According to the construction of FIG. 2, the monitoring voltage Vm is amplified to a voltage Vma, and it becomes possible to adjust the monitoring voltage signal Vma to a level suitable for comparative control with the optical-emission level control signal Vc.
On the other hand, the operational characteristic of a laser diode depends on the operational temperature thereof, and thus, the foregoing feedback control has to be accompanied with a temperature correction by providing a temperature sensor for detecting the temperature of the laser diode LD.
However, the use of such a temperature sensor in the laser driver circuit increases the cost of the apparatuses that uses such a laser driver circuit and laser diode.
The present invention has an object of providing a laser control apparatus for stabilizing the output of a laser diode without using a temperature sensor.